This invention relates to an electromagnetic driving device that operates an engine valve for an internal combustion engine of an automobile. This type of the electromagnetic driving devices have been proposed, for example the prior art disclosed in Japan unexamined publication (Koukai) No.8-21220, in which an intake valve slidably disposed on a cylinder head is operated by a magnetic force. Referring now to FIG. 12, there is an electromagnetic driving device that includes a driving mechanism 103 that controls the opening and closing status of an intake valve 102. The intake valve 102 includes a valve portion 1002a that opens and closes an opening of an intake port 104 and a valve stem 102b that is integrated with an upper end of the valve portion 102a. The electromagnetic driving mechanism 103 comprises an armature 106 that is encased in a casing 105 fixed on the cylinder head 100, is shaped like a plate, and is couple with the upper end of the valve stem 102b, and a pair of electromagnets. In particular, a closing-electromagnetic 107 and an opening-electromagnetic 108 are housed in the respective upper and lower portion of the casing 105 and are placed opposite side of the armature 106 for attracting the armature 106 so as to open and close the intake port 104. An armature spring 109 is installed between an upper wall of the casing 105 and an upper surface of the armature 106 and urges the valve 102 to close the opening of the intake port 104. Similarly, a valve spring 110 is interposed between a concave portion 101 of the cylinder head 100 and a lower surface of the armature 106, and urges the valve 102 to close the opening of the intake port 104. Further, each coil of the closing and opening-electromagnets 107, 108 is energized through an amplifier 111 according to a control signal produced by an electronic control unit 112. The electronic control unit 112 produces the control signal that controls a power-supply to the closing and opening-electromagnets 107, 108 based on any of plural detected signals, such as an engine revolution detected by an engine revolution sensor 113 and a temperature of coolant detected by the thermometer 114. Also, a power source designated 115 supplies a power to each coil of the closing and opening-electromagnets 107,108. Thereby, the spring forces of the valve and armature spring 109,110 and the attracting forces of the electromagnetic 107,108 are retained in respective springs 109,110 as a potential energy. With this, the valve 102 is alternatively attracted and released by one of the closing and opening electromagnets 107,108 to open or close the opening of the intake port 104 using that potential energy.
In the prior art, however, the attracting force of the closing and opening-electromagnets 107,108 is larger than the spring forces of the armature and valve spring 109, 110 urging the armature 106 against respective attracting forces, so that the end of the valve portion 102a might collide with a valve seat 104a of the cylinder head when the valve 102 closes the opening of the intake port, and the armature 106 might collide with the opening-electromagnetic 108 when the valve 102 opens the opening of the intake port. Referring to FIG. 13A, 13B, there is shown the principle of the attracting force for the closing and opening-electromagnets 107, 108. First, FIG. 13B shows the characteristics of the spring force between the attracting force of the electromagnets 107,108 and the spring forces of the armature and valve spring 109,110 when the valve 102 closes the opening of the intake port, the armature 106 is attracted to the closing-electromagnetic 107. Thereby, the valve spring 110 is extended and the armature spring 109 is compressed, so that a spring force of the armature spring 109 is accumulated therein. Next when the valve 102 opens the opening of the intake port, the closing electromagnet is de-energized, and alternatively the opening-electromagnet 108 is energized, thereby the armature 106 is attracted to the opening-electromagnetic 108. Therefore, the armature spring 109 is extended and the valve spring 110 is compressed, so that a spring force of the valve spring 110 is accumulated therein. Namely, respective accumulated forces of the armature and valve spring 109, 110 affects the armature 106 to reduce the speed of the armature 106 approaching the electromagnets 107,108. On the other hand, because the attracting force of the electromagnet is increased indirectly by the second power of a distance between the core of the electromagnets 107a, 108a, the attracting forces of the electromagnets are suddenly increased when the armature 106 is close to the electromagnets. Therefore, since the accumulated spring forces are not large enough to reduce the increased attracting force of the electromagnetic, the armature 106 is suddenly urged toward the one of the electromagnets 107, 108 when the armature 106 approaches close to one of the electromagnets 107, 108. As seen in FIG. 13A, due to a sudden increased attracting force of the electromagnets 107,108, the valve 102 collides with the valve seat 104a, when the valve closes the opening of the intake port, and the armature 106 collides with the opening-electromagnet 108, when the valve 102 opens it. Thereby, such a collision causes not only noise but also might cause wear and a fracture of the armature 106 and the valve seat 104a.
Moreover, the prior art requires an arrangement that balances the force between the attracting force of the closing-electromagnet 107 and the armature spring 109, in order to make the valve portion 102a urged toward the valve seat 104a with an appropriate force. However, since a gap between the armature 106 and the core of the electromagnet 107a is varied because of wear of the armature 106, the valve springs 109, 110 and the valve seat 104a and a heat expansion of the valve stem 102b, thereby the attracting force is also varied from the required value. Thus, the gap will occur between the valve portion 102a and the valve seat 104a, so that the valve portion 102a might not be capable of closing the opening of the intake port 104 tightly and might be covered with foreign matter. If so, the valve also might be melted since the foreign matter prevents dissipating heat therefrom.
Also, method of assembly of the prior art structure of FIG. 12 will be described. At first the valve 102 is inserted in the cylinder head 101 from the opening of the intake port 104a. Then, the opening-electromagnet 108 is disposed on the cylinder head 101. Finally, the armature 106 is coupled with the end of the valve stem 102b with installing the armature and valve springs 109, 110. Therefore, the prior art requires that assembly has to be performed using on the cylinder head 101, and an accurate arrangement of upper and lower position of the armature 106 with respect to the electromagnets 107, 108. Accordingly, the prior art increases the cost of the manufacturing and manufacturability.